1. Field of the Invention
The present invention relates to a spiral fabric papermakers felt, and more particularly to a dryer felt in the form a low permeability spiral coil fabric having stuffer materials positioned within certain of the coils to provide a fabric having a desired permeability profile in both the machine and cross-machine directions.
2. DESCRIPTION OF THE PRIOR ART
A conventional dryer felt or fabric consists of an open ended fabric, joined by means of a seam to form an endless belt, typically made from a two, three, or more plane fabric, wherein the various planes are defined by different groups of cross-machine direction yarns. The planes, plies, or layers, as they are variously called, are united by a plurality of machine direction yarns.
The yarns used to weave the most up-to-date dryer fabrics are generally made from synthetic monofilaments, synthetic multifilaments, or a combination of both from materials such as polyester, polyamide, acrylic, fiberglass or aramid fibers such as Kevlar and Nomex manufactured by E.I. duPont de Nemours and Co. Dryer felts made predominately from monofilament yarns have certain drawbacks. Because the monofilament yarns are relatively stiff, they are not easily bent around each other during the weaving process, and thus the fabric that results has a relatively open structure. There are several positions on the papermaking machine that do not run or cannot run effectively when employing a very open fabric because of numerous problems with the paper sheet, such as thread-up, blowing, and flutter, which causes sheet breaks, and reeling problems.
A number of attempts to reduce the openness or permeability of dryer fabrics made predominantly of monofilaments have been made. One approach has been to use a bulky spun yarn as a cross-machine stuffer pick in the middle of the weave pattern. These stuffer picks are, in effect, surrounded by the original monofilament cross-machine direction picks that are positioned in both the face and back surfaces of the fabric. This approach has been successful in reducing permeability, but has not substantially improved the stability of the fabric. Additionally, the spun stuffer pick is prone to collect dirt and has a tendency to retain and carry moisture, a condition which is undesirable in a dryer fabric.
A second approach to reduce fabric permeability has been to modify the woven structure in such a way that the top, or face, cross-machine direction picks are offset in relation to the bottom, or back, cross-machine direction picks. Although this approach has produced relatively low permeability in an all monofilament fabric, there is no easy way to change permeability because the weave design does not permit the use of stuffer picks. Changes in yarn diameter are, of course, possible, but such changes can only be made within the limitations of the loom.
An additional approach to the control of permeability in a dryer felt involves the incorporation of warp yarns of rectangular cross section into a weave pattern that does not include provision for stuffer picks. In such a weave pattern, the warp or weft yarn typically floats on the paper-receiving surface of the fabric over a number of weft picks or warp ends. The longer the float, i.e., the more picks the warp yarn crosses, or the more ends the weft yarn crosses, before weaving back into the fabric, the less stable the fabric becomes. In this way, there is a tradeoff between permeability and fabric stability.
In addition to woven fabrics, certain types of non-woven structures have been employed as dryer felts or fabrics. Of particular interest in connection with the present invention are those non-woven structures made from a plurality of intermeshed spiral coils that extend in side-by-side relationship in the cross-machine direction, and in which adjacent coils are joined together by cross-machine direction hinge yarns.
As is presently known, the predominant approach to reducing permeability in such spiral fabrics involves filling the gap within a given spiral coil created when that spiral coil is secured by hinge yarns to two adjacent spiral coils. Typically the gap is filled with a stuffer-type yarn. Another approach uses smaller spirals in an attempt to reduce the size of the space within a given coil.
In the first approach, the stuffer yarns are usually inserted as an extra production step after the basic fabric has been manufactured and finished. Although permeability is reduced, fabric processing time is increased and, therefore, this approach is less economical. At the same time, the use of stuffer yarns tends to reduce the clean running of the fabric and also reduces its ease of cleaning as dirt will rapidly adhere to the stuffer yarn.
The use of smaller spirals, on the other hand, necessitates increasing the number of filling yarns (which act as hinge yarns) per unit length. This again reduces productivity and increases costs. Also, it has been observed that the reduction in permeability is relatively small, such fabrics, at best, having a permeability of 800 cfm or more.
In addition to reducing the overall permeability of dryer fabrics, it is also desirable to provide such fabrics having different permeabilities across their widths. For example the paper sheet may have less moisture at the edges than at the center and it is therefore desirable that the edges of the fabric have a lower permeability than the center so that the drying effect on the paper sheet is less at the edges and more in the center. By the use of such dryer fabrics, problems such as grainy edges, sheet curl, dry edges, uneven moisture profile, and poor reel condition can be minimized or eliminated.
Similarly, if the paper sheet has a lower moisture content at the center than at the edges, then it is desirable to have a lower permeability in the center of the dryer fabric. Again, fabrics having variable permeability will help to eliminate such problems as wet edges, uneven moisture profile, and poor reel condition.
A woven fabric having different permeabilities across its width is not a new concept, and some early woven fabrics were designed with increased machine direction ends in the low permeability areas. This type of design reduced the load per machine direction end in the closed area compared with the open area and led to problems of fabric instability because the open area was prone to stretch more than the closed area. More recent designs have used increased numbers of cross-machine direction yarns in the closed area thus overcoming the disadvantages noted above when machine direction yarns were increased. However, while this approach overcomes one problem it can create another in that increasing the number of cross-machine direction yarns increases the machine direction yarn crimp in the closed areas compared with the open areas. The increased crimp in the machine direction yarns can cause the closed areas to stretch more than the open areas when subjected to the same load, sometimes resulting in slack edges to the fabric.
There is thus a need for a dryer felt of spiral coil construction that can be easily and economically produced, that has a wide permeability range which can be accurately controlled, that is stable and also dirt resistant, and that exhibits reduced moisture carrying properties. The present invention is directed toward filling that need.